Sheetmetal 9 inch diff

[1320ft]

One of the turnoff's for most guys that look at an IRS 9 inch is the cost. You can buy a Pro Mod rearend for the same price of the Driveshaft shop IRS 9" kit.

IRS is essentially a pointless exercise here in aus, because you need to have floaters faster than 7.50et.

[Supercrown]

Having a think about it Bill, I suspect that when you said that you get the same reaction with De Dion tube you were thinking that the angling of the links is purely what provides separation or 'anti squat' of the suspension.

The thing is that in a traditional 4-link solid rear axle set-up with the typical drag geometry of the links (the link geometry is critical - without it the force I'm describing does not occur), the reaction to the rearward rotation of the diff housing is a downward force on the diff housing. IE - if you were to jack the rear of the car up off the ground, and apply the rear brakes to stop rotation of the tyres, application of the throttle will cause the diff housing to move downward and away from the body. This reaction is quite separate to the forces caused by the forward force applied to the diff housing caused by the rearward force on the ground applied by the tyres. That's just the basic concept - of course getting the forces balanced properly on launch is a combination of spring rates, link lengths and angles, upper and lower diff end link distances from the axle centreline, damping rates, but the separation force is a critial component and without it the system won't generate the same level of traction on launch. If you set up two rear ends - a traditional one and de-dion one - both using identical link positioning and geometry, the traditional one will be able to impart a higher normal force on the tyres at the throttle hit from a stationary start.

Have a crack at a few vector diagrams using different link angles and diff end link distances from the axle centreline and you'll see what I'm getting at.

Of course in a 4-link solid axle circuit car you don't want the same level of separation force on throttle application (amongst other things), and the angled links play havoc with rear steering geometry with unequal suspension compression. When I was driving the Crown around on the street with the home job 4-link that had drag optimised link angles the back end moved all over the place as I went through corners that had undulations. With the new rear end set-up I'll be able to have the links in drag geometry for the strip, and then alter them to parallel for street duties.

[Billzilla]

Having a think about it Bill, I suspect that when you said that you get the same reaction with De Dion tube you were thinking that the angling of the links is purely what provides separation or 'anti squat' of the suspension.

Yeah I didn't think it through - got it wrong.
Ta for the info.